The present invention relates to the field of balloon catheters and more particularly to catheter balloons having controlled failure mechanisms for the prevention of catastrophic failure of the balloon during overpressure conditions.
Balloon catheters are used for a variety of medical procedures. Their conventional use entails the insertion of the balloon catheter into a body conduit at a cannulation site and pushing the length of the catheter progressively into the body conduit until the balloon located at the distal end of the balloon catheter reaches the desired site. The balloon is then inflated at that site in order to implement the desired therapy. The body conduit is most often a blood vessel and more particularly an artery, although balloons are used within a variety of other body conduits such as, for example, bile ducts. The inflation of the balloon may be used for various therapeutical reasons such as causing temporary occlusion of the body conduit, for the delivery of a medicant to the specific site of inflation, to disrupt plaque or thrombus or to deliver a device to a desired site within the body conduit. Devices most commonly delivered with a catheter balloon include vascular stents, vascular stents in combination with vascular grafts (stent-grafts), and intraluminal vascular grafts, all of which may be circumferentially distended by inflation of the balloon until the device is implanted in firm contact with the wall of the body conduit.
In use, catheter balloons are known to occasionally rupture due to inflation to higher than design pressures. Sudden rupture and corresponding sudden release of inflation pressure has resulted in damage to the surrounding body conduit. Even if the balloon remains intact following rupture the configuration of the damaged balloon may make withdrawal of the balloon from the body conduit quite difficult. These occasional ruptures can also result in fragmentation of the balloon and the necessity to retrieve the fragments. Due to displacement of the fragments distally as a result of fluid flow through the body conduit, retrieval is difficult at best and may require interventional surgery. It may not be possible to know with certainty that all pieces have been retrieved.
Provision for the rupturing of catheter balloons is known. U.S. Pat. No. 5,221,258 to Shturman teaches a catheter balloon having a longitudinal segment of weakness intended to allow the intentional rupture of the balloon between its ends in order to release a device contained within the balloon. Shturman also describes that the balloon may be provided with a portion of larger diameter than the remainder of the balloon in order that the higher hoop stress in the larger diameter portion results in rupture, again in order to release a device contained within the balloon.
Catheter balloons of various different designs have different normal operating pressures which are not intended by the manufacturer to be exceeded. If these normal operating pressures are exceeded by more than the amount of safety margin designed into a particular balloon, then the balloon is liable to rupture between the balloon ends. This amount of pressure required to result in failure by rupture is commonly referred to as the burst or rupture pressure. Rupture may result in fragmentation of the balloon with the fragments being difficult or impossible to recover, potentially leading to serious health problems for the patient. Likewise, failure by rupture can result in the damaged balloon being very difficult to remove due to the balloon material dragging on the vessel walls. Rupture may also result in improper deployment of a device being delivered by the balloon, requiring removal of both the device and the damaged balloon. All of these types of failure may be considered to be catastrophic.
The balloon catheter of the present invention provides for non-catastrophic failure by means other than rupture. This is accomplished by allowing an end attachment of one end of the balloon to fail by any of various means at a predetermined inflation pressure that is between the normal maximum operating pressure and the burst pressure. The predetermined pressure is not required to be a precise specific inflation pressure but may be a range extending for any portion of the range of pressure between the normal maximum operating pressure and the burst pressure.
The present invention thus relates to a catheter balloon having a controlled failure mechanism such that release of inflation pressure contained within the balloon occurs in a controlled fashion when a predetermined pressure is exceeded within the balloon. The controlled failure is the result of incorporating a failure mechanism into at least one end of the balloon (e.g., with a controlled amount of attachment of the balloon end to the catheter shaft). The result, as described above, is that the attachment of the at least one end fails at a predetermined pressure with the failure occurring before catastrophic rupture of the balloon between its attached ends. The end having the controlled failure mechanism may be either the distal end (leading end) or the proximal end (trailing end). Regardless of which end is made to be weaker resulting in failure of that end, the balloon remains intact and is not separated from the catheter shaft following failure, and is therefore easily withdrawable without loss of fragments within the body conduit.
Alternatively, both ends of the balloon may be provided with the controlled failure mechanism if there is no reason to limit failure to a particular end. With failure occurring at either end, the balloon remains intact and joined to the catheter shaft, and easily withdrawable from the body conduit into which it has been inserted.
The controlled failure mechanism is most preferably restricted to the balloon ends in the region of the attachment of the balloon end to the exterior surface of the catheter shaft. In normal operation below the predetermined controlled failure pressure, the controlled failure mechanism does not extend into the region of the balloon between the attached ends that is directly exposed to the fluid used to supply the inflation pressure. In this way, uncontrolled rupture of the balloon between the attached ends is avoided. With increasing pressure leading ultimately to the predetermined pressure at which it is desired to cause failure in a controlled manner, the stress on the balloon end attachments to the exterior surface increases and is utilized to provide the balloon failure in a controlled manner at the attachment site.
For purposes of the present invention, the balloon ends (or end attachments) are the end portions of the balloon which are affixed to the catheter shaft by any suitable method. During normal use, the ends of the balloon are not directly exposed to the inflating medium (such as a saline fluid) used to supply inflation pressure to the interior of the balloon between the opposing ends.
Various embodiments entail the provision of at least one aperture or slit at least partially through the thickness of the balloon material in at least one end region of the balloon where it is secured to the exterior of the catheter shaft. Excess pressure results in progressive failure of the end attachment with the result that the inflation pressure is relieved through exposure of at least a portion of the aperture or slit. Two or more slits located at one end of the balloon may be used. This arrangement allows for one side of the end of the balloon (between the slits) to come free from its attachment to the catheter shaft. The result is the release of the balloon pressure in a controlled and more gradual manner than would be the case with conventional catastrophic rupture, while maintaining the integrity of the remainder of the balloon in order that the failed balloon catheter is easily removed in its entirety.
It is apparent that the various methods of providing the controlled failure mechanism for specific balloon designs in order to achieve failure at desired predetermined pressures will require engineering with regard to all construction aspects of the balloon catheter. These aspects include materials selection, number of layers provided, method of attachment of the balloon to the catheter shaft, and various dimensions. They also include the possibility of controlling the rate and direction of fluid flow following the controlled failure, by the use of failure mechanisms such as the various types described herein. Such engineering is within the capability of ordinary skill in the art.
The design of catheter balloons and the use of reliable materials have resulted in a state of development at which it is possible to predict the failure of a balloon by rupture with some accuracy. With the controlled failure mechanism of the present invention, it is also possible to reliably cause failure to occur in a non-catastrophic manner at a predetermined pressure which is less than the pressure at which the balloon could be anticipated to rupture catastrophically.
A preferred balloon for use with the failure mechanism of the present invention is described by U.S. Pat. Nos. 5,752,934 and 5,868,704 to Campbell et al., incorporated by reference herein. These patents teach the construction of a composite balloon preferably made from porous expanded polytetrafluoroethylene (hereinafter ePTFE) and an elastomeric material such as silicone or polyurethane. As further described by these patents, the ends of the composite balloon are preferably secured to the exterior surface of a catheter shaft using a wrapping of a narrow tape of ePTFE film retained by an adhesive such as cyanoacrylate adhesive. The manufacture of ePTFE, including the films from which the composite balloon is manufactured and which are also used to secure the balloon ends to the catheter shaft are made, is taught by U.S. Pat. Nos. 3,953,566 and 4,187,390 to Gore. These patents are also incorporated by reference herein.